Year: 2026
Type: article
Abstract: High-harmonic generation (HHG) is conventionally understood to produce attosecond pulse trains only when the emitted harmonics are mutually phase-locked, in accordance with the classical theory of mode-locking. In a quantum optical framework, pulse synthesis would also require a regular phase relation between the components, when the harmonic field is in a separable multimode quantum state. This requirement, however, can be circumvented for harmonic combs exhibiting intermodal entanglement. We introduce a novel family of multimode quantum states, termed attoquants: states associated with highly structured pulse trains that are inherently insensitive to relative phases. As a concrete example, we analyse the coherent permanent state, constructed as a completely symmetric superposition of products of coherent states over all permutations of a fixed parameter set. It is shown analytically that its electric-field mean value is a locked Fourier superposition of the driving-field harmonics, yielding an attosecond pulse train without requiring regular phase relations between the high-harmonic components. We further calculate the photon statistics, the Wigner function, and the logarithmic negativity of these states, in order to show their genuinely nonclassical character, and the degree of entanglement. Finally, we offer a phenomenological interpretation for the generation of such entangled states during HHG, as a consequence of multi-atom effects. Since the coherence volume of the driving field greatly exceeds that of its harmonics, a collectively-driven cluster of atoms can radiate the harmonics as spatially separated, but classically indistinguishable sources. This can generate the permutation-symmetric entanglement structure of an attoquant.
Source: New Journal of Physics
Authors: Sándor Varró, Ákos Gombkötő
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $2,610
OpenAlex ID: W7203750746